Method of magnesium production



Sept. 28, 1943.

R. H. M CARROLL EI'AL METHOD OF MAGNESIUM PRODUCTION Filed July 24, 1941 lV/IWGl/V 1L6 IVA TEE WALTER POT LOA DING UN]. 04 DING C 00L I Ne mwfawm Patented Sept. 28, 1943 METHOD OF MAGNESIUM PRODUCTION Russell H. McCarroll, Dearborn, Frank G. Shaub,

Detroit.

and Joseph S. Laird. Dearborn,

Mich, assignors to Ford Motor Company, Dearborn, Mich, a corporation of Delaware Application July 24, 1941, Serial No. 403,834

9 Claims.

This invention relates to the method of producing magnesium and like metals; and, more particularly, to a method in which the oxide of such metals is electrothermally reduced.

An object of this invention is to improve the method of carrying out the reduction reaction. A further object is to provide a more efficient method of handling the gaseous products of the reaction to lessen its reversability and increase the precipitation therefrom. Still another object is to remove the slag and similar by-products of the ieaction from the furnace and automatically dispose of them. Another object is to remove the hazards incident to other refining systems and to so carry out the process that the material obtained at intermediate steps therein may be safely handled and stored.

We make use of a reduction reaction which has been widely studied; and our apparatus, which comprises, basically a furnace and a condenser, may also superficially appear to have its counterpart in the prior art. But both method and apparatus have been fundamentally altered, as we shall point out; and it is by these changes that the objects sought are attained.

The electrothermic reduction of an oxide of magnesium by carbon will result as in the following equation:

MgO+C Mg+CO It is generally known that in the above reversible reaction, magnesium metal can be obtained by sharp quenching through the critical temperatures. In the usual procedure, magnesium vapors are quenched by means of large amounts of a cooled nonoxidizing gas, usually hydrogen. With this procedure the metallic magnesium is. condensed from the vapor phase to a finepowder form, some of which is carried away with the cooling gases. Not only is magnesium extremely hazardous in this powder form, but also great difllculty is experienced in obtaining coales- I, cence of the fine powder. This metal, regardless Figure 1 is a diagrammatic flow sheet of the method.

Figure 2 is an elevation of the furnace and cone used in the method, a portion thereof being broken away to show the interior construction.

Referring to Figure 2 of the accompanying drawing, the furnace iii and associated cone 6! are shown. In the preferred form of construction, the cone is rigidly mounted, while the furnace is equipped with'rollers so that it may be swung away from the cone to permit access to the interior. The furnace comprises an outer shell !2 which is generally cylindrical in shape and which encloses the refractories which, in turn, form a furnace chamber Hi. This chamber extends upwardly to a stack l4 through the roof of the furnace, the electrode l5 being. placed therein.

A conventional electrode feed it governs the position of the electrode and thisis also equipped with means for supplying cooling water around the electrode and with other means by which nitrogen may be directed into the stack.

One side of the furnace is indented, substantially in the form of a frustum of the cone, as at H, forming a port which communicates with the tap hole It. This decreases the distance between the arc and the quenching point.

In the preferred form of construction, the main furnace block I9, the first stack block 20 and the hearth block 2| are of carbon, which is less reactive than usual magnesia refractories. The tap-hole block 22 i carborundum or zirconia and the lining 23 of the port is stainless steel. These inner blocks are surrounded by a number of outer layers of refractory material, such commercial products as Silocel, Insulbrix, Magnoline or. Magnesite having been found suitable. The upper stack block must be, of course, an insulator.

The hearth block 2i is in the form of a frustum of the cone and has. a central hole 25 therethrough, the diameter of this hole increasing toward the chamber. The hearth block is held down on the floor 26 of the furnace by a surrounding steel cone 2! which is secured to the fioor. Aligned with this hearth hole is a steel nozzle 28, which is also secured to the floor and which communicates with the vertical ram 30. A horizontal ram 3| leads thereto, and a conveyor 32 directs material to the latter to be charged into the furnace. Thus, material fed into the conveyor proceeds through the horizontal ram and is compacted and forced therefrom into the vertical ram. The latter'then compacts and forces the material upwardly through the nozzle riods.

and the hearth block. Considerable pressure is required in this operation and, as a result, substantially all of the air is removed from the material during the feeding operation. The force involved is so large that it has been found necessary to taper the hearth hole as indicated in the drawing'.

In the furnace shown, a single-phase electrical system is employed, the electrode I5 forming one element thereof and the charge in combination with the hearth block 2i the other. As the material is reduced, the magnesium in the'form of a gas escapes through the tap hole It to the cone l l, communicating therewith.

The cone II is substantially a condensing chamber, having a front cone portion 33 anda cylindrical portion at. The embodiment shown consists of a double-walled chamber having the exterior wall 35 and an interior wall 36. In the space 37 between theseilwo walls, cooling oil is circulated to lower the temperature of the cone.

A central shaft 38 run through the cone and is sufficient to produce 100 pounds of magnesium is shown. The process starts with the charge of into-the furnace it which is supplied with 1500 the interior of the cone when desired.

The shaft also supports a scraper mechanism 52 having blades 33 which scrape the inner surfaces of walls 33. A plurality of oil jets 94 extends through the walls of the cone and at their inner ends have a nozzle 45, designed to atomize and direct the quenching oil therein inwardly of the cone as indicated by the lines 46. This oil is expelled at high velocity as a fine mist and fills the interior of the cone. may be placed adjacent to each oil jet or may be made concentric therewith. The purpose of these latter jets is to. introduce nitrogen into the interior of the cone before starting operations and to build .up the pressure there-to prevent leakage of air into the cone during inactive pe- They also permit the removal of the oil jets for cleaning, a temporary seal of nitrogen being created.

The cylindrical portion 34 has as its upper part the explosion diaphragm 48 and as its lower part the delivery sump 49 from which the magnesiumcontaining oil flows, I

In operation, the gaseous magnesium enters the portion 33, where it is struck'by the oil spray and is sharply quenched. As much of this oil is recirculated, a certain amount of condensed magnesium is already present therein. The magnesium vapor is quenched and thereby condensed to metallic magnesium which is carried by the oil to the inner 'wallsof the cone. Some of the magnesium is deposited there in the form of a light mud and is scraped therefrom by the scraper blades 43. The slary then flows down the cone into the sump in t cylindrical portion 34 from whence it flows for further treatment.

The furnace and cone have thus been described in some detail. In Figure 2 they are shown in operating positions, that is, the cone abutting the tap hole of the furnace. In this position, the

Nitrogen jets 3i cubic feet of nitrogen. There, the magnesium oxide is reduced and the magnesium in the form of a gas passes into the cone l I, where it is condensed by an oil spray 46. The condensed magnesium, now in a light oil slurry, flows from the cone sump 49 (by a screw conveyor 5!) to a seal 52, which permits the gas in the mixture, amounting to approximately 200 pounds of which 40 per cent is CO and the remainder nitrogen and hydrogen, methane, etc, resulting from cracking of the quenching oil, to be piped to a holder while the oil and magnesium slurry flows to a settling tank 53, a suitable vent 56 being provided. The screw conveyor is provided to remove any materials that might settle or cling between the aforementioned points. The purpose of the seal is to separate the gas from the oil as it is discharged from the cone and at the same time prevent the leakage of air back into the cone. At this point, the slag maybe separated out.

The slurry is held in the tank 53 and the magnesium powder permitted to settle. The heavier portions of the slurry are carried through a filter 55 and thence to a suitable drier 55, although the latter can be dispensed with if desired. The

' pot is carefully sealed by welding a top thereto,

'and a vacuum connection placed thereto. The

pot is then placed within ,the furnace 59 and the pressure within suitably reduced while heat is' applied. The oil distills off first, the vacuum is increased to .1-.5 cm. Hg, absolute, and the magnesium is then vaporized and condensed in the upper portion of the pot which is outside of the distilling furnace 59. After from 16-20 hours, the pot is removed from the distilling furnace and is cooled. Thereafter the top of the pot may be removed and 100 pounds of magnesiumin a bright, metallic, crystalline formation is disclosed.

.The material remaining on the rack, which consists .of unreacted magnesia and carbon, oil residue, ash,slag, etc., is ground up and returned to the furnace as the residue referred to below. The quantities noted on the flow sheet are illustrative and ane based solely on a desired output of 100 pounds of magnesium per hour. .It will be understood that the furnace and cone will operate continuously and a considerable output,

' bon and a residue. Although fairly large lumps of carbonaceous material were tried and used, it was found that materials pulverized to mesh were most suitable. Theresidue is that material resulting from previous 'heats which was unreacted or reoxidized during the quenching process and which remained after the distilling operation. In this manner, loss of magnesium-containing material is kept at 'a minimum and the highest yield of magnesium per raw materials used is obtained, The materials fed to the electm -reduction furnace are proportioned to provide the necessary carbon for reaction with the magnesia present and vary with the amount and character of the residue. As an example, we may use one part of carbon, one part of residue and four parts of MgO.

An intimate mixture of these materials is fed continuously through the bottom of a furnace by means of the system of hydraulic rams. These rams are 'used because the conventional screw feeding devices stalled or twisted out of shape when the materials compacted. In addition, the feeding sleeve had to be tapered to relieve some of the resistance.

The furnace we use is unique inasmuch as the raw materials make up one of the electrodes and hearth. While shown in combination with an oil quenching device, it may be used with other quenching means such as hydrogen, lead, etc. The other electrode, a carbon rod, is synchronously fed from the top of the furnace to the reduction zone.

' The packing of the raw material is essential. Removal of all air therefrom and the maintaining of a seal through the charging apparatus are thus made possible; and explosive hazards from this source are eliminated. Also, the compressed charge of raw materials becomes the lower electrode since it is compacted by pressures as large as 15 tons per square inch and becomes a conductor of electricity. Moreover, the charge becomes the greater portion of the hearth, as may be seen in Figure 2. The charge is injected just beyond the hearth block where the arc occurs, a small pool of fused magnesia forms, and the magnesium oxide is reduced. The material is kept at a proper level by means of an interlocking device between the feeding ram and the movable electrode. The hearth block itself only burns out to certain limits and then seems to reach an equilibrium, thereafter remaining unchanged. The electrical circuit is completed through the compressed raw materials by an electrical connection to the hearth block.-

The means for sealing the furnace from the outside atmosphere must be faultless. The use of a nonoxodizing and inert (insofar as magnesium is concerned) gas such as hydrogen has been tried. But hydrogen is itself potentially dangerous and its explosive characteristics are greatly increased by the powdered magnesium obtained.

We have found that nitrogen, while supposedly reactive with magnesium, in fact does not react therewith at the high temperatures obtaining in this process and to form a stable compound yields results superior to those obtained with hydrogen when used as a seal. As nitrogen is easily and safely handled, much of the danger in an already dangerous process is avoided. The nitrogen is introduced just below the packing that seals the electrode at the place of entrance to the furnace. Suflicient gas is fed to this system to produce a seal although the pressure in the furnace will not exceed 2#/in. This requires approximatel 50 to 120 cubic feet of nitrogen perpound of magnesium. Feeding nitrogen in the above manner also forces the-vapors from the furnace into the condensing chamber. Moreover, the velocity of the gas stream down the furnace stack prevents the condensation of magnesium on the electrode which invariably results in a danserous short circuiting. Another feature attributed to nitrogen is the prevention of excessive re-oxidation. That is, nitrogen causes an interference between magnesium vapors and carbon monoxide so as to produce greater yields of magnesium metal by reducing the tendency of reformation of magnesium oxide and carbon.

The construction of the condensing chamber or cone must be efficient, safe and foolproof. In prior practice, the cones utilizing hydrogen or other gaseous quenching mixtures had to be purged several days before using to insure an oxygen-free atmosphere, otherwise conditions might exist that would jeopardize the whole plant. In this process the reduction furnace and the cone are two separate units, and the furnace can be moved so that repair or other labor may be accomplished on both or either as the conditions may dictate, meanwhile maintaining the desired atmosphere in both. However, the distance between the furnace arc and the quenching zone should be short for best results.

Gaseous quenching methods require large amounts of cooled gases as the specific heats are low. It is evident that a liquid such as an oil cools or quenches magnesium vapors much more sharply. Furthermore, oil,'after being clarified,

can be re-used, sustaining only a small loss. Quenching gas, such as hydrogen, can only be used once, since the concentration of CO becomes so great that the reaction might proceed to the left too rapidly. With this mind, a jacketed cone was constructed with jets for both a gas and an oil, the jacket to aid in maintaining the required temperature differentiation between the cone and furnace. The cone can be sealed when shutting down or when the furnace is removed and, under such conditions, a positive pressure of nitrogen may be applied. In this manner, resumption of production may be accomplished within several hours because the system does not require lengthy purging, whereas hydrogen systems require several days for this.

The oil used at present in both jacket and cone is a light mineral oil a little heavier than kerosene and having a viscosity of approximately 42 seconds at F., Saybolt Universal. The jacket and quenching spray have separate oil circulatoil is not necessary and proves impracticable since production of five pounds of magnesium per hour requires circulation of from 300 to 1,800 gallons of oil per hour. We have not only found that by re-circulating the quenching oil; the burden on the clarifying system is relieved, but also that such re-circulated oil is a more eflicient quenching medium. The suspended solids serve as nuclei for the condensation of magnesium, and the production is increased.

These suspended solids, of course, must be controlled so that concentrations do not become high enough to clog the spraying nozzles. These solids consist of magnesium metal, re-oxidized magnesium oxide, carbon and impurities of the process that may have been blown or forced into the cone from the furnace. We continuously clarify aportion of the oil suspension, maintaining an equilibrium of suspended magnesium. This clarification may be effected by various means. For instance, centrifugal separators. pressure filters, and machines of the principle of the Dorr thickener may be used. The amount of solid magnesium removed from an oil suspension for a given time oftreatment in any one of these devices may be readily determined and the treating time adjusted accordingly to obtain the desired separation from the suspension.

' This quantity is susceptible of very close adjust-' ment and it will be found that the time chart of the process may be so adjusted as to obtain a practically uniform concentration of magnesium remaining in the oil which is recirculated t the quenching cone. While the separators and the filter are essentially intermittent in action while the thickener is continuous, yet as a practical consideration the quenching oil may beconsidered as being continuously recirculated, since it is immediately taken from the first two devices (if either of these is used) and restored to the circulating system.

The impurities of the process referred to above chiefly consist of the slag of the raw materials.

pure metal.

We claim as our invention:

1.'In a process of producing metallic magnesium comprising, the reduction of MgO by C, sharp quenching the reduction products, including Mg vapors, to metallic magnesium and attendant impurities to a solid slag in anoil spray, removing said slag from the oil-magnesium slurry so formed, collecting'said magnesium from nesium, said magnesium vapor being condensed thereby to form a slurry with said oil, concentrating the magnesium content of said slurry'to the oil into a thick mud, and distilling said mud under reduced pressure. L v

2. The process of producing metallic magnesium by first reducing MgO in the reaction M O+C2Mg+CQ and then sharp quenching the magnesium vapors to metallic magnesium by means of an atmosphere of atomized oil, said atomized oil having been re-circulated and containing a finely-divided metallic magnesium suspended therein.

3'. The process of quenching magnesium vapor which includes the steps of subjecting said vapor to a violently agitated atmosphere of 'atomized oil, thereby condensing said vapor in the form of finely-divided metallic magnesium, suspending said magnesium in said oil to form a slurry,

' settling said slurry to obtain a sludge in which the greater part of the suspended magnesium is concentrated and alight oil suspension of the remaining magnesium, re-circulating said light oil suspension and atomizing it to form the quenching atmosphere, the finely-dividedparticles of said finely-divided magnesium in said atmospherefacilitating the condensation of the magnesium 'vapor therein.

4. A process of producing metallic magnesium comprising, the carbothermal reduction of MgO to magnesium vapor and CO, quenching said vaporin an oil spray containing as an additional quenching medium finely divided metallic magform a thick mud, subjecting said mud to reduced pressures and high temperature to separate theoil therefrom and to sublime the metallic magnesium to reguline magnesium.

5. A process of producing metallic magnesium comprising, the reduction of MgO to form magnesium vapor and CO, mimng therewith an atmosphpre of nitrogen, condensing said magnesium vapor in said atmosphere to finely divided metallic magnesium by an oil quenching spray, concentrating said condensed magnesium into a thick mud, and treating said mud to form reguline magnesium.

6. A process of producing metallic magnesium comprising, the reduction of MgO to form magnesium vapor and C0, mixing therewith an atmosphere of nitrogen, condensing said magnesium vapor while in said atmosphere to metallic magnesium by intermingling therewith an oil spray, separating impurities incident to the reaction from said oil, concentrating said condensed magnesium in said oil to form a thick mud, and treating said mud to form reguline magnesium.

7. In the process of reducing magnesium oxide by the reversible reaction MgO+CZMg+CQ comprising the steps of forming gaseous Mg and C0 and biasing the reversibility of said reaction by mixing with said gaseous product an atmos-' phere of nitrogen whereby the tendency of the reaction to reverse to the left is diminished.

8. A process of producing metallic magnesium by carbothermal reduction, comprising the steps of feeding an intimate mixture of magnesium- -containing material and carbon through a' reaction chamber, compressing said mixture as it is supplied to said chamber to remove the air therefrom and to seal said compressed mixture with respect tothe chamber to prevent the entrance of air therein, subjecting said compressed and deaerated mixture to a temperature sufiicient to induce the carbothermal reaction, removing the products of said reaction, including magnesium vapor and CO, from the proximity of said arc, mixing therewith nitrogen at substantially the temperature of the reaction products,

and substantially immediately quenching said va- Y tween said material and the entrance into said reaction chamber, subjecting the upper surface of said charge to a temperature suificient to melt said magnesium-containing material'and form a pool thereof at the surface of said charge, said temperature being high enough to induce the carbothermal reaction to form magnesium vapor and CO, removingsaid magnesium vapor from a said arc and sharp-quenching it to form finely divided metallic magnesium.

1 RUSSELL H. MCCARROIL.

FRANK G. SHAUB. JOSEPH S, LAIRD. 

